The present invention relates to hydraulic control valves and more particularly to a sectional valve having dual pressure relief.
Sectional hydraulic valves utilize multiple spools to control the various functions performed by hydraulic cylinders on forklift trucks, end loaders and similar machines. Typically, a single spool will control a cylinder for a particular function such as hoist (lift), tilt or an auxiliary function.
In lift truck applications it is desirable to make available fluid pressure for the hoist or lift function that is greater than the fluid pressure needed for the tilt or auxiliary functions. Typically, a hoist or lift pressure of 2,000 psi is desirable while a tilt or auxiliary function pressure of 1,900 psi is desirable.
Valve systems such as those shown in the U.S. Pat. Nos. 3,324,881 and 3,324,882 patents to Keir utilize a pair of pilot valves that are operable at different pressures and which in turn cause operation of a main relief valve.
While the dual pressure relief valve system of the Kokaly U.S. Pat. No. 3,416,561 patent shows the use of a single relief valve which operates at two different pressures. The Kokaly relief valve is used to monitor two sides of a single circuit (raise and lower).
The Murase et al. U.S. Pat. No. 3,811,471 patent also utilizes two pilot relief valves and one main relief valve.
The prior art has also obtained dual pressure relief by utilizing a special external line which will deliver fluid at high pressure in the hoist circuit when desired. All of the above methods and arrangements require additional parts, i.e. pilot valves or external circuitry which add to the cost of the valve.